The present invention relates to NCO-terminated prepolymers which are stable to sedimentation, to a process for the production of NCO-terminated prepolymers that are stable to sedimentation, and to their use. These NCO-terminated polymers are based on polyether carbonate polyols.
As known and described in the prior art (U.S. Pat. No. 4,808,691), polyether carbonate polyols based on hexanediol ether are reacted with polyisocyanate in a molar excess, in most cases with 4,4′-diphenylmethane diisocyanate, to give isocyanate prepolymers having terminal NCO groups (“NCO prepolymers” or “NCO-terminated prepolymers”). These NCO-terminated prepolymers are, in turn, important raw materials for the production of polyurethanes and particularly for polyurethane cast elastomers.
Polyether carbonate polyols are predominantly bifunctional, hydroxyl-group-terminated polyols which contain both carbonate groups and ether groups. In accordance with the prior art, polyether carbonate polyols are prepared in a condensation reaction using, for example, diphenyl carbonate or dimethyl carbonate or carbonic acid dichloride, mixtures of ether-group-containing diols, which are prepared by acid catalyzed condensation of diols, and unetherified diols being used.
A disadvantage of the above-mentioned NCO-terminated prepolymers prepared from 4,4′-diphenylmethane diisocyanate and hexanediol ether carbonate polyols is that they form a sediment when stored at room temperature. Increasing the storage temperature to prevent sediment formation is of no help because the viscosity then rises considerably, and as a result, the NCO-terminated prepolymers ultimately become unusable. The same is also true if once cooled and hence sediment-containing NCO-terminated prepolymers are heated again. On further reaction, the precipitate that forms leads to inhomogeneities in the finished casting elastomer, which in turn can be a cause of material failure.
The object of the present invention was, therefore, to produce NCO-terminated prepolymers in a manner such that a precipitate no longer forms, or settles out, during storage of the prepolymer at room temperature.
It has now surprisingly been found that NCO-terminated prepolymers based on polyether carbonate polyols can be so produced such that they are stable to sedimentation if one or more specific polyols are mixed with the polyether carbonate polyol before the reaction with 4,4′-diphenylmethane diisocyanate.